Method for producing malleabilized castings



United States Patent IVIETHOD FOR PRODUCING MALLEABILIZED CASTINGS Lon Saives, Billanc'ourt, France, assign'or to Regie Natlonale des Usines Renault, Billancourt, France No Drawing. Application February 10, 1953 Serial No. 336,188

Claims priority, application France February 21, 1952 6 Claims. (Cl. 148-3) In my co-pending patent application Serial No. 335,579 of February 6, 1953, I have described a process which makes it possible to obtain industrially cast irons of great toughness with nodular or spheroidal graphite, starting With pieces of ordinary thickness, cast in sand and having a white structure (cementite structure). It has now been discovered that these treatments are greatly facilitated, that is to say, they can be carried out much more quickly and produce a much finer structure, the graphite being in grains which are very fine and very numerous, if the cast iron is solidified in chill-moulds. Such a process of manufacture is doomed to failure, however, as a result of the blow-holes, shrinkage holes, and above all the microshrinkage-holes which appear in the chill-casting of the iron.

On the other hand, it is known that die-casting in chill-moulds, though suitable for fusible alloys of zinc, aluminum, magnesium and copper, is not suitable for steels, chiefly because of the tendency of the liquid steel to deteriorate in the storage furnace and the difficulty of obtaining metal chill-moulds which can withstand the action of the jet of metal at a very high temperature.

It is also known that die-casting of iron has not been carried out to date because of the impossibility of obtaining high mechanical characteristics, either because the cast iron is in the form of lamellar graphite which does not transmit the tensile strains or the shearing strains, or because the cast iron is in the form of extremely fragile dendritic cementite.

It has now been discovered that the combination of the processes of chill-casting, die-casting, and graphitization with pre-treatment for the nuclear formation, makes it possible to combine their advantages without running the risk of these drawbacks. This combination leads industrially to new products of high quality, high output of mass-produced parts, precision and good surface qualities in the castings, high strength of the cast iron, and regularity of production.

In fact, the die-casting or iron in chill-moulds is greatly facilitated, compared with steel, by the great stability of the metal bath in the storage furnace, by the lowering of the casting temperature, and by the considerable reduction in the effects of hot erosion on the chill-mould.

Thus white iron can be cast without micro-shrinkagecracks or blow-holes, thanks to the pressure exerted during solidification. Cracks are avoided by stripping immediately after solidification is finished.

Due to the chill-casting, the iron can have a relatively high silicon content which lowers the tapping temperature while preserving a white structure in the rough-cast pieces. Thus the normal silicon content in malleable cast irons can be considerably exceeded, reaching values of between 1.5 and 2%. Moreover, higher carbon contents of 2.6 to 3% can be allowed for the same reasons.

In order to obtain the optimum effect, it is preferable, according to the invention, to use irons containing copper although this is not an absolute necessity. Copper acts as a constituent to improve the casting qualities, as an ICC agent for the nuclear formation of the graphite, and as a suitable additive for the hardening and tempering heattreatment. The quantity of copper is suitably 0.4 to 3% preferably 1 to 2%.

In order for the process to have its full elfect, it is necessary, according to the invention, to carry out the triple treatment of hardening, nuclear formation and graphitization. The hardening treatment can be carried out, starting from the casting heat, by stripping fairly hot, above 810 C. and quenching directly in a salt-bath, for example at 180 C. for 1 minute, then cooling instill air, which makes it possible to obtain chilled castings without the risk of cracks or shrinkage cracks. If the casting comprises narrow parts which have been ever-cooled, it can, after being stripped, be immersed in a stabilizing bath at 810 C. one minute, after being salt-hardened at 180 C. The casting is then subjected to nuclear formation treatment at a temperature within the range of 400 to 500 C. for a period of time within the range of 5 to hours, for example 36 to 48 hours, e.g. 48 hours, at 450 C., then, with or without intermediate cooling, the casting is subjected to graphitization of the primary cementite. In order for the graphitization to be absolutely complete, it is necessary to maintain it at 875 C. for 2 to 6 hours, but this time can be reduced to between 40 minutes and two hours by annealing at 900 C. In general, graphitization is carried out at a ter'nperature within the range of 850 to 900 C. for a period of time within the range of 20 minutes to l2 hours. The following table identifies the percentage composition in carbon, silicon, manganese, and copper and gives the number N per square millimeter of fine spherules of graphite having an average diameter of 2 to 6 microns obtained by this process. In these tests, the chill-casting was 14 mm. thick. Stripping was done at more than 820 C. and the piece was hardened directly in salt at C. The annealing comprised a first cycle of 48 hours at 450 C., and a second of 3 hours at 875 C.

N o. of casting O Si Mn Cu N /mm.

In order for the process to attain its full eifectiveness, it is preferable to allow the piece to cool upon removal from the mould and then to reheat it for austenization at 810 C. for 30 minutes, for example, and hardening in stages, at 180 C. for 1 minute for example. The castings are then subjected to nuclear formation treatment, for example 48 hours at 450 C., cooled, then reheated to 875 C. for example, just long enough for the graphitization of the primary cementite, then cooled in still air, failing which, if the time taken is too long, the graphite undergoes a coalescence with a reduction in the number of spherules and a lowering of the mechanical properties. This is shown by the following table which gives the number of spherules of graphite per square millimeter having a diameter of less than 2 microns:

In each case, the quantity of cementite is nil, so that the annealing for 1 hour at 875 C. is sufiicient. It will be seen that here the best result is obtained with the No. of casting Ou E,kg./mm. R,kg./mm. A

In the foregoing table E represents the elastic limit of the castings expressed in kg./mm. R is the tensile strength expressed in kg./mm. and A is the elongation expressed in percentage.

After this treatment, a piece of casting 2162 was reheated to 835 C., hardened in oil, tempered at 700 C.; it then showed: E280, R=82, A=2.5% in a machined test-piece 4 mm. in diameter.

It is also possible, after this last tempering at 700 C., to stop this by oil-hardening and temper for 2 hours at 500 C., to induce the structural hardening of the copper.

Finally, it is possible to modify the compositions and properties of these cast irons by alloys such as: Ni, Mo, Ti, Al, Zr and the like. The machinability, after graphitization, with or without hardening and tempering, is particularly easy, due to the graphite.

I claim:

1. A method of obtaining malleable iron of pearlitic structure containing diffused fine particles of graphite which comprises die casting in a chill mold an iron suitable for the formation of pearlitic malleable iron, stripping the casting from the mold while hot, subjecting the casting to austenization with heat at a temperature of about 810 C., quenching the casting in a salt bath at a temperature of about 180 C., effecting nuclei formation of graphite by subjecting the casting to a temperature between 400 to 500 C. for 5 to 100 hours, and elfecting graphitization of the primary cementite by heating the casting to a temperature of 850 to 900 C. for 20 minutes to 12 hours.

2. A method of obtaining malleable iron of pearlitic structure containing difiused fine particles of graphite which comprises die casting in a chill mold an iron suitable for the formation of pearlitic malleable iron, stripping the casting from the mold while hot, subjecting the casting to austenization with heat at a temperature of about 810 C. for 30 minutes, quenching the casting in a salt bath at about 180 C., for about one minute, cooling the casting in still air, effecting nuclei formation of graphite by subjecting the casting to a temperature between 400 and 500 C. for 5 to hours, and effecting graphitization of the primary cementite by heating the casting to a temperature of 850 to 900 C. for 20 minutes to 12 hours.

3. A method of obtaining malleable iron of pearlitic structure containing diffused fine particles of graphite which comprises die casting in a chill mold an iron suitable for the formation of pearlitic malleable iron, stripping the casting from the mold while hot, subjecting the casting to austenization with heat at a temperature of about 810 C., quenching the casting in a salt bath at a temperature of about 180 C., cooling the casting in still air, effecting nuclei formation of graphite by subjecting the casting to a temperature of about 450 C. for 36 to 48 hours, and eifecting graphitization of the primary cementite by heating the casting to a temperature of 850 to 900 C. for 20 minutes to 12 hours.

4. A method of obtaining malleable iron of pearlitic structure containing difiused fine particles of graphite which comprises die casting in a chill mold an iron suitable for the formation of pearlitic malleable iron, stripping the casting from the mold while hot, subjecting the casting to austenization with heat at a temperature of about 810 C., quenching the casting in a salt bath at a temperature of about 180 C., cooling the casting in still air, eliecting nuclei formation of graphite by subjecting the casting to a temperature of about 450 C. for 36 to 48 hours, and eifecting graphitization of the primary cementite by heating the casting to a temperature of 875 C. for about one hour.

5. A process as defined in claim 1, wherein the iron treated contains 0.4% to 3% copper.

6. A process as defined in claim 1, wherein the iron treated contains 1% to 2% copper.

References Cited in the file of this patent UNITED STATES PATENTS 1,498,128 Sowers June 17, 1924 2,185,894 Hultgren Jan. 2, 1940 2,331,886 Boegchold Oct. 19, 1943 2,564,885 Sternberg Aug. 21, 1951 OTHER REFERENCES Transactions of the American Foundrymens Association, vol. 50, 1942, pp. 1052, 1953.

Materials and Methods, vol. No. 32, issue No. 6, December 1950, pp. 50-53. 

1. A METHOD OF OBTAINING MALLEABLE IRON OF PEARLITIC STRUCTURE CONTAINING DIFFUSED FINE PARTICLES OF GRAPHITE WHICH COMPRISES DIE CASTING IN A CHILL MOLD AN IRON SUITABLE FOR THE FORMATION OF PEARLITIC MALLEABLE IRON, STRIPPING THE CASTING FROM THE MOLD WHILE HOT, SUBJECTING THE CASTING TO AUSTENIZATION WITH HEAT AT A TEMPERATURE OF ABOUT 810* C., QUENCHING THE CASTING IN A SALT BATH AT A TEMPERATURE OF ABOUT 180* C., EFFECTING NUCLEI FORMATION OF GRAPHITE BY SUBJECTING THE CASTING TO A TEMPERATURE BETWEEN 400 TO 500* C. FOR 5 TO 100 HOURS, AND EFFECTING GRAPHITIZATION OF THE PRIMARY CEMENTITE BY HEATING THE CASTING TO A TEMPERATURE OF 850 TO 900* C. FOR 20 MINUTES TO 12 HOURS. 